This article is focused on Magento2 default themes, responsive design, implementation of new theme and uninstall theme from Magento2; helpful for frontend theme developers.
In Magento, theme is a component of Magento application which provides a consistent look and feel for entire application area using a combination of custom templates, layouts, styles or images. Magento2 provides two themes Luma and Blank. Luma is known as demonstration theme, where as Blank theme is the basis for custom theme creation. Though there is no restriction to use these default themes but in case of customization, the best practice is to create new theme package. Editing default theme is not recommended because changes can be overwritten by the new version of the default files during upgrades.